As a result of the changed market economic conditions in the open electricity supply markets and the improved technologies in the field of power electronics, the subject of variable speed drives for energy production has gained in importance. For this purpose, doubly-fed asynchronous machines are preferably used, especially in the case of outputs of above 60 MVA.
The stator of this type of machine does not differ from those of salient pole synchronous machines which are used for this application. Machines of this type are characterized in that they are equipped with a three-phase winding both on the stator as well as on the rotor. In this case, the end windings of the rotor winding are customarily arranged on a cylindrical surface (see for example DE-A1-195 13 457).
As a result of centrifugal force, the winding bars have the tendency to move away from the center radially outwards during operation. The difficulty in the case of said construction of the end windings is in sufficiently protecting the bars of the winding against deformation as a result of mechanical forces. For this, three solutions are generally made available:                In the case of smaller machines, the entire end winding is wound with a plurality of layers consisting of steel wire. For production engineering reasons this is not advisable in the case of larger diameters.        In the case of machines with diameters of more than 2 meters, the end windings are fastened on the axial extension of the rotor yoke either by means of radial bolts or threaded rods with a support, as is described in the aforementioned publication, or        The entire end winding is enclosed by a shrunk-on steel cylinder, as is known in the case of non-salient pole synchronous machines.        
The aforementioned publication concentrates particularly on the description of the overall construction with the associated end winding support system. The bolts which are used in this case serve for the radial support of the rotor end windings and are anchored in the axially raised rotor yoke (see FIGS. 4 and 5 of the publication with the bolts 37). The bolts are threaded rods which on the one hand are screwed (36) in the rotor yoke, and on the other hand, on the air-gap side, absorb the resulting centrifugal forces by means of rectangular individual elements and nuts. The disadvantage of the described system lies in the large number of individual parts and the associated risk of destruction of the machine when losing a bolt or a nut.
The development which is described in the following text deals in more detail with the bolts which are used and especially with their secure fixing.